but i got one circuit digram of mosfet based UPS from EFY . They have given as "using a single battery of 12v,7Ah with a load (100-120W) comprising computer along with color monitor the back up time is 10-15 min with square wave output (half of sine wave output).
with battery of 12v,180Ah the back up time is 4-5hrs with square wave output (2-2.5hrs with sine wave output)."